1. Field of the Invention
This invention relates to a method for noise reduction and stabilization of the welding arc in TIG-ac-welding machines with self-commutated inverter current sources.
2. Description of Prior Art
TIG-ac-welding machines with self-commutated inverter current sources, such as are disclosed by German Patent DE 31 50 510 A1, make it possible to pre-select the alternating frequency of the welding current. This technology has only become economically feasible due to rapid power semiconductors, which have only recently become available. The following points should be considered in the choice of the correct alternating frequency:
A dc welding arc has negligible sound emissions with acceptable current quality. However, commutation processes continuously occur during ac welding. During this commutation process, the current becomes zero and returns to its original value, but with reversed polarity. Also occurring is a pole reversal of the magnetic field to which the arc is subjected as ionized plasma. The plasma moves in this magnetic field like an almost inertialess diaphragm in a loudspeaker and thus emits noises. High-speed flow changes occur with high welding currents causing strong changes in the magnetic flux and resulting in the sound emission level of the ionized welding arc becoming very high. Furthermore, the absolute level of the emitted noise is directly proportional to the alternating frequency because such noise-generating commutation processes occur correspondingly more often.
Research using test persons has shown that, with a given welding current, the subjective noise level actually does increase substantially with the alternating frequency and is felt to be penetratingly disturbing at frequencies around 200 Hz.
From the perspective of environmental damage, it is not recommended to operate a TIG-ac-welding machine at a high frequency, for example 150 Hz, simultaneously with a high welding current, for example 250 A or more. However, the quality of the arc also must be considered.
Modern machinery is expected to achieve satisfactory welding results even in the range of a few Amperes in connection with thin sheet metal. Ac welding machines are overwhelmingly used for welding of aluminum, the great disadvantage being that a highly temperature-resistant oxide layer forms on the surface of the metal. This oxide skin is destroyed by a commutation of the welding current as well as the use of a high-voltage ignition device which penetrates the oxide skin with its short, but energy-rich pulses. With low welding currents, the amount of energy supplied to the arc is comparatively low. Tests have shown that during welding with low currents, the selection of a high alternating frequency is advantageous. The more frequent polarization changes have a beneficial effect on the arc and on the cleaning of the aluminum surface. With high currents, there is so much energy contained in the arc that this aspect is of no importance.
The present state of the art provides a step switch or a potentiometer for the welding machine by which the operator can pre-select the alternating frequency.
However, this solution has some disadvantages. With each change of the pre-selected current, the operator is required to adjust the frequency at the same time. Often less qualified persons who do not know the fundamental operation may work with a disadvantageous setting for a long time. Furthermore, in a TIG-machine, the welding current is not a constant, but rather continuously changes during normal operation. Initially, there is an increasing ramp, that is, the current begins low, for example, with 20% of the end value, and increases during a pre-selected time to the end value. In the same way there is a decreasing ramp of, for example, four seconds at the end of welding. The machines also provide the possibility of pulsing. This means that, in selectable time intervals, the welding current is switched between the highest value and a selectable lower value, for example, 40% of the highest value. If the operator operates with a foot pedal, as is still mostly the case, he can regulate the current individually.
Accordingly, the fixed setting of the alternating frequency by means of a separate knob does not take into account in any way the continuously changing current value.